Continuous production of ethylene homopolymers

ABSTRACT

PRODUCTION OF ETHYLENE HOMOPOLYMERS HAVING A SPECIFIC DENSITY BY POLYMERIZATION OF ETHYLENE UNDER THE ACTION OF ORGANIC PEROXIDES AND OXYGEN AS FREE-RADICAL-GENERATING POLYMERIZATION INITIATORS AND OF POLYMERIZATION MODIFIERS AT ELEVATED TEMPERATURE AND SUPERATMOSPHERIC PRESSURE IN A TUBULAR REACTOR HAVING TWO SUCCESSIVE REACTION ZONES, A MIXTURE OF ETHYLENE, POLYMERIZATION INITIATOR AND POLYMERIZATION MODIFIER BEING INTRODUCED CONTINUOUSLY AT THE BEGINNING OF EACH REACTION ZONE. OXYGEN AS POLYMERIZATION INITIATOR AND A MIXTURE OF A POLYMERIZATION MODIFIER HAVING A SPECIFIC RELATIVELY LOW C5 VALUE AND A POLYMERIZATION MODIFIER HAVING A SPECIFIC RELATIVELY HIGH C5 VALUE ARE USED IN THE FIRST REACTION ZONE, AND AN ORGANIC PEROZIDE HAVING A SPECIFIC HALF VALUE TEMPERATURE AS POLYMERIZATION INITIATOR AND A MIXTURE OF A POLYMERIZATION MODIFIER HAVING A SPECIFIC RELATIVELY LOW C5 VALUE AND A POLYMERIZATION MODIFIER HAVING A SPECIFIC RELATIVELY HIGH POLYMERIZATION MODIFIER HAVING A SPECIFIC RELATIVELY HIGH C5 VALUE ARE EMPLOYED IN THE SECOND REACTION ZONE. THE ETHYLENE HOMOPOLYMERS OBTAINED HAVE A WIDE MOLECULAR WEIGHT DISTRIBUTION AND ARE PRACTICALLY DEVOID OF VERY HIGH MOLECULAR WEIGHT CONSTITUENTS.

United States Patent US. Cl. 260--94.9 R 6 Claims ABSTRACT OF THEDISCLOSURE Production of ethylene homopolymers having a specific densityby polymerization of ethylene under the action of organic peroxides andoxygen as free-radical-generating polymerization initiators and ofpolymerization modifiers at elevated temperature and superatmosphericpressure in a tubular reactor having two successive reaction zones, amixture of ethylene, polymerization initiator and polymerizationmodifier being introduced continuously at the beginning of each reactionzone. Oxygen as polymerization initiator and a mixture of apolymerization modifier having a specific relatively low C value and apolymerization modifier having a specific relatively high C value areused in the first reaction zone, and an organic peroxide having aspecific half value temperature as polymerization initiator and amixture of a polymerization modifier having a specific relatively low C,value and a polymerization modifier having a specific relatively high Cvalue are employed in the second reaction zone. The ethylenehomopolymers obtained have a wide molecular weight distribution and arepractically devoid of very high molecular weight constituents.

The present invention relates to a process for the continuous productionof ethylene homopolymers having a density of from 0.922 to 0.926 g./ccm. by polymerization of ethylene under the action of oxygen andorganic peroxides as free-radical-generating polymerization initiatorsand of polymerization modifiers at reaction temperatures of from 280 to320 C. and pressures of from 1500 to 4000 atmospheres in a tubularreactor which has two successive reaction zones of about equal length, amixture of ethylene, polymerization initiator and polymerizationmodifier being continuously introduced into the reactor at the beginningof each reaction zone.

The following is typical for prior art methods of this type, cf. forexample, French patent specification Nos. 1,202,623 and 904,000 and thepublished papers of Belgian Patent No. 943,980:

A mixture of ethylene, a polymerization initiator having a relativelylow half value temperature and a polymerization modifier is introducedinto the first reaction zone where the reaction temperature is keptrelatively low. A mixture of ethylene, a polymerization initiator havinga relatively high half value temperature and a polymerization modifieris introduced into the secondreaction zone Where-the reactiontemperature is kept relatively high. In this way it is possible toobtain products having a wide molecular weight distribution. This isdesirable for example in the case of ethylene homopolymers which are PCC to be processed into laminating film or sheet because a widemolecular weight distribution is accompanied, inter alia, by a lowneck-in (formation of a bulge at the edges of the film while it is stillcapable of plastic flow after extrusion from a sheeting die). Theproducts not only have a wide molecular Weight distribution but also afairly large fraction of very high molecular weight constituents. Thelatter may be a great disadvantage, for example in the case of ethylenehomopolymers which are to be used in the 'field of laminating film; theyresult, inter alia, in the molten polymer having a poor draw-down i.e.in the extrusion of film and sheeting, there is an increasing tendencyfor flaws (specks and holes) to form as the thickness of the filmdecreases.

The object of the present invention is to provide a process of the typedefined above with which it is possible to prepare ethylene homopolymerswhich not only have a wide molecular weight distribution but also arepractically devoid of very high molecular Weight constituents.

We have found that the said object is achieved by using oxygen aspolymerization initiator and a mixture of a polymerization modifierhaving a specific relatively low C value and a polymerization modifierhaving a specific relatively high C value in the first reaction zone andan organic peroxide having a definite half value temperature aspolymerization initiator and a mixture of a polymerization modifierhaving a specific relatively low C value and a polymerization modifierhaving a specific relatively high C value in the second reaction zone.

Accordingly, the present invention relates to a process for thecontinuous production of ethylene homopolymers having a density of from0.922 to 0.26 g./ccm. by polymerization of ethylene under the action ofoxygen and an organic peroxide as free-radical-generating polymerizationinitiators and of polymerization modifier at reaction temperatures of280 to 320 C. and pressures of from 1500 to 4000 atmospheres in atubular reactor which has two successive reaction zones of about thesame length, a mixture of ethylene, polymerization initiator andpolymerization modifier being introduced continuously into the reactorat the beginning of each reaction zone.

The process according to the invention comprises:

1) Introducing at the beginning of the first reaction zone a mixture ata temperature of from 170 to 200 C. and a pressure of from 1500 to 4000atmospheres which contains an amount by weight equivalent to from 5 tomole ppm. (with reference to the ethylene) of oxygen, 0.1 to 1.0 percentby weight of a polymerization modifier having a C value of 1.5 10- to1.0 10- and 0.1 to 0.7 percent by weight of a polymerization modifierhaving a C value of 25x10 to 4X1O- and keeping the reaction temperaturein said zone within the range from 300 to 320 C.; and

(2) Introducing at the beginning of the second reaction zone a mixtureat a temperature of to C. and a pressure of from 1500 to 4000atmospheres which contains an amount by weight equivalent to from 3 to20 mole ppm. (with reference to the ethylene) of an ora (3,, value of2.5 10- to 4 10- and keeping the reaction temperature in said secondreaction zone within the range from 280 to 305 C., with the proviso thatthe ratio by weight of the mixture introduced per unit time into saidfirst and second reaction zones if from This process enables ethylenehomopolymers having a density of from 0.922 to 0.926 g./ccm. to beprepared which not only have a wide molecular weight distribution butalso are practically devoid of very high molecular weight constituents.

Conventional tubular reactors are suitable for carrying out the process.The process may also be carried out in a conventional manner provideddue regard is had to the peculiarities according to the inventionoutlined above. Rather than give further details, reference is made forexample to UK. patent specification No. 934,444.

The oxygen to be used in the process is preferably used in the form ofair.

The organic peroxides to be used according to the invention should havespecific half value temperatures. By half value temperature We mean thetemperature at which half of a given amount of peroxide has decomposedafter one minute.

Specific examples of organic peroxides having a half value temperature(hvt.) within the range from 150 to 195 C. are tert butyl peracetate(hvt.=154 C.), tertbutyl perbenzoate (hvt.=166 C.), ditert-butylperoxide (hvt.=193 C.) and methyl ethyl ketone peroxide (hvt.=182 C.).It has been found that ditert-butyl peroxide is particularly suitablefor the purposes of the present invention. It may be advantageous to usethe peroxides in the form of solution in inert solvents in theconventional manner.

The polymerization modifiers to be used according to the inventionshould have specific C values (cf. G. A. Mortimer, J. Polymer Sci.,A/1,4, 881 (1966) for the concept and definition of C value). Specificexamples of suitable polymerization modifiers having relatively low Cvalues are methane (C value=7 10- propane (C value=3.1 10- andcyclohexane (C value=7.7 X10 It has been found that propylene (Cvalue=1.3 10 and propane (C value=3.1 10 are particularly suitable forthe purposes of the invention. Specific examples of polymerizationmodifiers having relatively high C values are n-butyraldehyde (Cvalue=4.0X l0 and cyclohexanone (C value=1.1 10'- It has been found thatpropioualdehyde (C va1ue=2.5 10 and methyl ethyl ketone (C value=2.7 10-are particularly suitable polymerization modifiers of this type for thepurposes of the invention.

The ethylene homopolymers obtainable by the process are particularlysuitable for the production of laminating film and for coating papersand fabrics.

The invention is illustrated by the following example.

EXAMPLE A conventional tubular reactor (jacketed for temperaturecontrol) having two successive reaction zones of equal length is used,the ratio of internal diameter to length being 1:10,000 in the firstzone and 1:9000 in the second zone.

A mixture at a temperature of 175 C. and under a pressure of 2300atmospheres which contains 3000 parts by Weight of ethylene, 0.29 partby weight of air (equivalent to 19.5 mole p.p.m. of oxygen withreference to the ethylene) and 21 parts by weight of propylene(polymerization modifier; C value=1.3 10- and 10 parts ofpropioualdehyde (polymerization modifier;

C value=2.5 10

is continuously introduced per hour at the beginning of 4 the firstreaction zone. The reaction temperature is kept at from 310 to 315 C.(a) by the heat of reaction liberated and (b) the jacket cooling.

A mixture at a temperature of 180 C. and under a pressure of 2300atmospheres which contains 3000 parts by weight of ethylene, 0.23 partby weight (equivalent to 7.8 mole p.p.m. with reference to ethylene ofditertbutyl peroxide (in the form of a 2% by weight solution in methylacetate; half value temperature=193 C.) and 21 parts by weight ofpropylene (polymerization modifier; C value=1.3 10- and 5 parts byWeight of propionaldehyde (polymerization modifier;

C value=2.5 X 10*) is introduced continuously per hour at the beginningof the second reaction zone. The reaction temperature is kept at from295 to 300 C.

The ethylene homopolymer obtained at the end of the second reaction zoneand the unreacted components of the reaction mixture are brought byperiodic lowering of the reaction pressure to 1600 atmospheres into aseparator which is at a pressure of 250 atmospheres.

In this way 1200 parts by weight per hour of ethylene homopolymer isobtained (equivalent to a yield of 20%); it has an intrinsic viscosityof 4.5 g./10 minutes and a density of 0.924 g./ ccm.; its neck-in isabout 7% and its draw-down is good.

We claim:

1. A process for the continuous production of ethylene homopolymershaving a density of from 0.922 to 0.926 g./ccm. by polymerization ofethylene under the action of oxygen and an organic peroxide asfree-radicalgenerating polymerization initiators and of polymerizationmodifiers at reaction temperatures of 280 to 320 C. and pressures offrom 1500 to 4000 atmospheres in a tubular reactor having two successivereaction zones of about equal length, a mixture of ethylenepolymerization initiator and polymerization modifier being introducedcontinuously at the beginning of each reaction zone, which comprises (1)introducing at the beginning of the first reaction zone a mixture at atemperature of from 170 to 200 C. and a pressure of from 1500 to 4000atmospheres which contains an amount by weight equivalent to from 5 tomole p.p.m. with reference to the ethylene of oxygen, 0.1 to 1.0 percentby weight of a polymerization modifier having a C value of 1.5 10- to1.0 10- and 0.1 to 0.7 percent by weight of a polymerization modifierhaving a C value of 2.5 l0- to 4X10- and keeping the reactiontemperature in said zone within the range from 300 to 320 C.; and

(2) introducing at the beginning of the second reaction zone a mixtureat a temperature of from to C., and a pressure of from 1500 to 4000atmospheres which contains an amount by weight equivalent to from 3 to20 mole p.p.m. with reference to the ethylene of an organic peroxidehaving a half value temperature in the range from 150 to 195 C., 0.1 to1.0 percent by weight of a polymerization modifier having a C value offrom 1.5 x10 to 1.0 10* and 40 to 60% by weight of the amount used insaid first reaction zone per 100 parts by weight of ethylene of apolymerization modifier having a C value of 2.5 10- to 4 10- and keepingthe reaction temperature in said second reaction zone within the rangefrom 280 to 305 C.,

with the proviso that the ratio by weight of the mixture introduced perunit time into said first and second reaction zones is from 1:2 to 2:1.

2. A process as claimed in claim 1 wherein said organic peroxide istert-butyl peracetate, tert-butyl perbenzoate, ditert-butyl peroxide ormethyl ethyl ketone peroxide.

3. A process as claimed in claim 2 wherein said polymerization modifiedhaving a C value of 1.5 10-'* to 1.0 10" is methane, propane,cyclohexane or propylene.

4. A process as claimed in claim 3 wherein said polymerization modifierhaving a C value of 2.5X10- to 4X10 is n-butyraldehyde, cyclohexanone,propionaldehyde or methyl ethyl ketone.

5. A process as claimed in claim 1 wherein said polymerization modifierhaving a C value of 1.5 10- to 1.0X10- is methane, propane, cyclohexaneor propylene.

6. A process as claimed in claim 1 wherein said p0- lymerizationmodifier having a C value of 25x10" to 4X10- is n-butyraldehyde,cyclohexanone, propionaldehyde or methyl ethyl ketone.

References Cited UNITED STATES PATENTS 3,142,666 7/1964 Deex et al.26094.9 5 3,293,233 12/1966 Erchak, Jr, ct al. 260-949 3,334,081 8/1967Madgwick et a1 260-943 OTHER REFERENCES Mortimer, G. A.: Journal ofPolymer Science, A/ 1,4, 10 pp. 882-888 (1966).

JAMES A. SEIDLECK, Primary Examiner A. HOLLER, Assistant Examiner

